The present invention relates to a solid state laser gain medium, and more particularly to such a medium for high power operation that has a diamond coating therein.
In a solid state laser having a gain medium, the medium is optically pumped to cause stimulated emission therein. When high power is generated, thermal gradients in the medium (the center being hotter than the outer portions) create circumferential (tangential) and axial tensile stress in the medium. The thermal stress will cause crack growth and possible fracture of the medium; surface damage cracks caused by finishing and handing accelerate this failure. In addition, a coolant, such as water, is normally flowing over and around the medium. This flow can cause stress corrosion of the medium, thereby enhancing the damaging effect of thermal stress.
Another problem is that of parasitic oscillations due to total internal reflections (TIR) at the boundaries of the medium. Such oscillations reduce the power output available from the laser and can also cause damage or cracking of the medium. It is known from the articles "Parasitic Oscillations and Amplified Spontaneous Emission in Face-pumped Total Internal Reflection Lasers," D. C. Brown et al., SPIE, Vol. 736, pp.74-83, 1987, and "Parasitic Oscillations, Absorption, Stored Energy Density and Heat Density in Active-Mirror and Disk Amplifiers," D. C. Brown et al., Applied Optics, Jan. 15, 1978, Vol. 17, No. 2, pp. 211-224, to leave some of the medium faces in a rough ground condition in order to reduce total internal refections and thus parasitic oscillations. However, this reduces the strength of the medium. It is known from U.S. Pat. No. 3,872,401 to use a glass (SiO.sub.2) coating or cladding on the gain medium to reduce parasitic oscillations. However, such coatings have insufficient durability when immersed in a flowing coolant due to abrasion and have reactively low tensile strength compared to the medium, which results in such coatings breaking up and flaking off. Such coatings also have low thermal conductivity, which results in a high thermal gradient, and therefore a higher thermal stress, and also have a higher thermal expansion than the medium so that they will eventually flake off if operated at a high power. It is also known from U.S. Pat. No. 3,508,165 to form both the medium and the cladding of lanthanum aluminate, but with different dopants, and thus different indices of refraction, to reduce parasitic oscillations. However such a cladding has relatively low corrosion resistance and tensile strength and a low thermal conductivity.
It is therefore an object of the present invention to both strengthen an optical gain medium and reduce parasitic oscillations therein.